Kaylen Holmesly, a 7th grade resident of Azle, Texas, testifies before the Texas Railroad Commission and voiced her concern about an increased number of earthquakes around Eagle Mountain Lake on January 21st, 2014.
Marjorie Kamys Cotera

*Correction appended

Gas industry activity “most likely” triggered a series of earthquakes that shook two North Texas towns from late 2013 through early 2014, new peer-reviewed research shows.

More than two dozen small earthquakes during that period rattled residents near Reno and Azle, towns atop the gas-rich Barnett Shale, and put pressure on Texas oil and gas regulators to address concerns about man-made temblors. A combination of industry activities likely caused the phenomenon, according to a study published Tuesday in the journal Nature Communications.

More specifically, according to the research, operators’ withdrawal of brine – naturally salty water removed during oil and gas drilling – and the high-pressure injection of huge volumes of wastewater from gas wells were to blame.

Mapping two intersecting faults in the area, scientists from Southern Methodist University, the U.S. Geological Survey and the University of Texas at Austin found that the interplay of those gas activities likely altered fluid pressure underground, unleashing the quakes.

“What we refer to as induced seismicity – earthquakes caused by something other than strictly natural forces – is often associated with subsurface pressure changes,” Heather DeShon, associate professor of geophysics at SMU, said in a statement. “We can rule out stress changes induced by local water table changes. While some uncertainties remain, it is unlikely that natural increases to tectonic stresses led to these events.”

Milton Rister, the Railroad Commission's executive director, told the researchers in a letter Tuesday that the research “raises interesting points along with many questions” and asked them for a briefing.

The study adds to the growing body of research that links disposal wells, and to a lesser extent some oil and gas production, to mostly small earthquakes. The number of disposal wells — deep resting places for liquid oil and gas waste — has surged amid Texas’ drilling bonanza. Drilling areas in South and West Texas have also seen more earthquakes.

Scientists have known for decades that injecting fluid deep underground could trigger earthquakes. Neighboring Oklahoma has seen an increase in earthquakes even greater than Texas has, and has surpassed California as the country’s most quake-prone state. The USGS and Oklahoma Geological Survey say wastewater disposal probably contributes to the trend.

The SMU-led research team said it developed new models allowing researchers to declare that gas activities are “most likely” causing the earthquakes, going beyond past studies that found them “possible” causes. The researchers also pointed out that thousands of disposal wells have not triggered earthquakes.

Though Texas, home to nearly 3,600 active commercial disposal wells, is four times the size of Oklahoma, it has far fewer seismic stations – devices that monitor quakes – positioned throughout the state. That makes it difficult to estimate the precise size and location of some quakes, and to understand what causes them.

To improve data on the quakes around Reno and Azle, the research team added several temporary seismic stations to the area in December 2013. The team used three-dimensional modeling to analyze the changing pressure in the rock formation from two wastewater injection wells and more than 70 production wells that produce natural gas and brine.

XTO Energy and EnerVest operate the injection wells examined in the study, David Pearson, the Railroad Commission’s seismologist, said in a conference call Tuesday afternoon.

Pearson said he needed more time to digest the study and meet with its authors, and would not recommend shutting down the wells – at least not yet.

“We will not be suspending activity at the two wells, especially given the fact that we have not seen any continuation of large-scale earthquakes in the Azle area that would give us any cause for alarm,” he said. “The swarm has died out and has been quiet for some time.”

The research team has been studying earthquakes in the Barnett Shale since 2008, when a cluster struck near DFW International Airport. Prior to that phenomenon, which hit early in the area’s drilling surge, no earthquake had been reported in the Fort Worth Basin since 1950.

But it was the Azle temblors – and the ensuing public outcry – that kick-started the Railroad Commission’s response to the phenomenon. The response included hiring Pearson – Texas' first state seismologist – and approving requirements that companies submit more information before drilling disposal wells. The Texas House formed a subcommittee on seismic activity.

Still, the agency's website says, “Texas has a long history of safe injection, and staff has not identified a significant correlation between faulting and injection practices."

Energy In Depth, a prominent industry-funded group, on Tuesday praised the researchers for developing a model that “provides greater understanding of the conditions that can ultimately lead to induced seismicity.” But on its website, it added that “several issues in the paper raise questions about its conclusion.”

“The concerns we’ve identified in here are in the spirit of constructive collaboration,” Steve Everley, the group’s spokesman, said in an email.

Pearson said the researchers “did a fantastic job” imaging the faults they examined, but said some of their assumptions and conclusions raised a number of questions. “We’re reviewing this, so it’s really hard for me to make a definitive statement about whether this dispels any concern about the relationship between wells and earthquakes.”

Correction: This story has been updated to note that Milton Rister, executive director of the Railroad Commission, signed a letter to researchers requesting a meeting. Craig Pearson, the agency's seismologist, did not sign the letter. The story has also been updated with information from a conference call with Pearson.

Disclosure: The University of Texas at Austin is a corporate sponsor of The Texas Tribune. Southern Methodist University was a corporate sponsor of The Texas Tribune in 2013. A complete list of Tribune donors and sponsors can be viewed here.

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